History and Use of Ultra-Violet Sterilisation
UltraViolet Germicidal Irradiation (UVGI for further reference) has been proven as an effective method to sterilise air, water and surfaces since the late 19th Century.
The sunlight was the first catalyst in this discovery in 1877 as Downes and Blunt, two English scientists realised that exposing test tubes containing Pasteur’s solution to sunlight inhibited the growth of bacteria and upon longer exposure could keep the test tube bacteria free for several months.
This discovery was since then qualified as “one of the most influential discoveries in all of photobiology”.
Some key factors were identified to assess the effectiveness of the UVGI performance.
The inactivation potential depends on the dose of radiation received (J x m2) which is itself the product of the intensity (W x m2) and the exposition in seconds. Finally it also depends on the wavelength of the radiation.
The shorter the wavelength, the more the radiation carries energy and thus, a threat to micro organisms.
Much of the later work was focused on identifying the most efficient wavelength for sterilisation purposes. The following wavelengths were studied and categorised:
Infrared (700 - 1000nm), Visible light (400-700nm), UV-A (315 - 400nm), UV-B (280 - 315nm) and UV-C (100 - 280nm).
Soon enough it was clear that UV-Cs were the most effective overall even though microbial sensitivity to light was different, the extent of inactivation required more or less exposure.
The first breakthrough in terms of applying the discovery to the real world happened when in 1933, William F. Wells theorised the concept of airborne infection by droplets nuclei and the use of UVGI to sterilise the air. Through sneezing or coughing it was proven that evaporated droplets containing nuclei could stay in the air for an extended period of time and present a risk of contamination for those around. He demonstrated in 1935 that aerolized B. Coli could be killed using 254nm UV-C GI.
Shortly after, it was proven that the use of UV-C lights included in in-duct HVAC (heating, ventilating, air-conditioning) was effective and could help sterilise hospital rooms.
Today, UV-Cs have regained popularity due to their cost effectiveness and a need to sterilise very crowded environments such as public transport. While UV-C indiscriminately affects micro-organisms and cells that are exposed to it (including human skin), UV lamps are now being used in a controlled environment to sterilise large areas and are much more effective than aerial sprays or wipes.